Exhaust gas purifying device and method for internal combustion engine

ABSTRACT

Removal of fine particles by oxidation or/and sulfur poisoning recovery control may be required when an internal combustion engine has been in an extremely low load state for a predetermined period or more. In this case, the engine speed of the internal combustion engine ( 1 ) is adjusted to a range where the temperature of a filter ( 20 ) can be raised by heat-up control. The heat-up control is then executed by a filter temperature control means to raise the temperature of the filter ( 20 ) to a predetermined value. When the filter ( 20 ) reaches the predetermined temperature by means of low-temperature combustion, post-injection, VIGOM-injection, addition of  10  fuel to an exhaust system and the like, removal of fine particles by oxidation or/and sulfur poisoning recovery control for eliminating sulfur poisoning of a NOx absorbent are conducted. Removal of PMs captured by the filter and sulfur poisoning recovery control of the NOx absorbent can thus be conducted even if the internal combustion engine is left in an extremely low load operational state.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to an exhaust gas purifying device and method foran internal combustion engine, and more particularly, to an exhaust gaspurifying device and an exhaust gas purifying method which are capableof conducting recovery from sulfur poisoning and the like even ifextremely low-load operation is continued.

2. Description of Related Art

In an internal combustion engine installed in an automobile or the like,especially in a diesel engine or a lean-bum gasoline engine in which amixture containing an excessive amount of oxygen (a mixture exhibiting aso-called lean air-fuel ratio) can burn, the advent of an technology forpurifying nitrogen oxides (NOx) contained in exhaust gas in the internalcombustion engine has been expected.

A technology of disposing an NOx absorbent in an exhaust system of aninternal combustion engine has been proposed to meet such a demand. Anocclusion/reduction-type NOx catalyst has been known as one type of theNOx absorbent. The occlusion/reduction-type NOx catalyst absorbsnitrogen oxides (NOx) contained in exhaust gas when the exhaust gasflowing into the catalyst exhibits a high oxygen concentration, anddischarges the absorbed nitrogen oxides (NOx) and reduces them tonitrogen (N₂) when the exhaust gas flowing into the catalyst exhibits adecreased oxygen concentration in the presence of a reducing agent.

In the case where the occlusion/reduction-type NOx catalyst is disposedin the exhaust system of the internal combustion engine, nitrogen oxides(NOx) contained in exhaust gas are absorbed by theocclusion/reduction-type NOx catalyst when the exhaust gas exhibits ahigh air-fuel ratio during lean-bum operation of the internal combustionengine, and the nitrogen oxides (NOx) absorbed by theocclusion/reduction-type NOx catalyst are discharged and reduced tonitrogen (N₂) when the exhaust gas flowing into theocclusion/reduction-type NOx catalyst exhibits a reduced air-fuel ratio.

It is to be noted herein that sulfur oxides (SOx), which are producedthrough combustion of sulfur contained in fuel, are also absorbed by theocclusion/reduction-type NOx catalyst according to the same mechanism asin the case of NOx. Sulfur oxides (SOx) are not discharged when nitrogenoxides (NOx) are usually discharged and reduced. Thus, if apredetermined amount or more of sulfur oxides (SOx) is accumulated, theNOx catalyst becomes saturated and unable to absorb NOx. Thisphenomenon, which is referred to as sulfur poisoning (SOx poisoning),causes a decrease in the NOx purification ratio. For this reason, apoisoning recovery process for recovering the NOx catalyst from SOxpoisoning needs to be performed at a suitable timing. This poisoningrecovery process is performed by allowing exhaust gas with a decreasedoxygen concentration to flow through the NOx catalyst while the NOxcatalyst is held at a high temperature (e.g., 600 to 650° C.).

However, exhaust gas is below the aforementioned temperature duringlean-bum operation. Thus, when the engine is in a normal operationalstate, it is difficult to raise the bed temperature of the NOx catalystto a temperature required for the recovery from sulfur poisoning. Insuch a case, it is possible to decrease the oxygen concentration ofexhaust gas while raising the temperature of the aforementioned catalystby adding fuel to an exhaust passage.

As a method for raising the temperature of the NOx catalyst, an exhaustgas purifying device for an internal combustion engine has been proposedin Japanese Patent Publication No. 2845056. The exhaust gas purifyingdevice for the internal combustion engine disclosed in this publicationdetermines the addition amount of a reducing agent in consideration ofthe amount of the reducing agent consumed through a reaction with oxygencontained in exhaust gas in an occlusion/reduction-type NOx catalyst andthe amount of the reducing agent required for the reduction of nitrogenoxides (NOx) absorbed by the occlusion/reduction-type NOx catalyst. Thisexhaust gas purifying device thus prevents the reducing agent from beingsupplied excessively or insufficiently and aims at inhibiting exhaustemission properties from being deteriorated by the discharge of thereducing agent or nitrogen oxides (NOx) into the atmosphere.

On the other hand, in a diesel engine, it is important to removeparticulate matters (hereinafter, referred to as “PMs” unless otherwisementioned) such as soot as a suspended particulate matter contained inexhaust gas. A technology of providing a particulate filter(hereinafter, simply referred to as “filter”) for collecting PMs in anexhaust system of the diesel engine in order to prevent the PMs frombeing discharged into the atmosphere is therefore well-known in the art.This filter collects the PMs contained in exhaust gas and thus preventsthem from being discharged into the atmosphere. However, if the PMscollected by the filter are accumulated on the filter, the filter may beclogged with the PMs. Such clogging raises the pressure of the exhaustgas upstream of the filter, thereby possibly causing reduced outputpower of the internal combustion engine and damage of the filter. Insuch a case, it is possible to remove the PMs by igniting and burningthe PMs accumulated on the filter. Such removal of the PMs accumulatedon the filter is called regeneration of the filter.

In order to ignite and burn the PMs collected by the filter, thetemperature of the filter must be raised to a high temperature of, e.g.,500° C. or more. However, since the exhaust gas temperature of thediesel engine is lower than this temperature, it is difficult to removethe PMs through combustion in a normal operation state.

It is possible to use an electric heater, burner or the like to heat thefilter to a temperature that causes ignition and combustion of thecollected PMs. However, this requires a great amount of energy to besupplied from the outside. Regarding this problem, Japanese PatentLaid-Open Publication No. 6-159037 and the like use a filter carrying aNOx catalyst and a device for supplying hydrocarbons to exhaust gas as areducing agent. This facilitates combustion of the PMs by using the heatgenerated by combustion of the hydrocarbons supplied to the exhaust gasin the NOx catalyst.

The aforementioned recovery from sulfur poisoning is carried out withthe oxygen concentration of exhaust gas decreased. However, if thereducing agent is added during high-load operation of the internalcombustion engine, the reducing agent bums in theocclusion/reduction-type NOx catalyst. As a result, the temperature ofthe occlusion/reduction-type NOx catalyst rises excessively. This maycause thermal degradation of the occlusion/reduction-type NOx catalyst.Accordingly, it is preferable that the recovery from sulfur poisoning becarried out while the internal combustion engine is in a low-load range.

However, in the case where the internal combustion engine is in anextremely low-load operation state for a long time, for example, in thecase where a vehicle having the internal combustion engine is parked inan idle state for a long time or runs in heavy traffic in a town, theinternal combustion engine discharges a small amount of exhaust gas andthus the absolute amount of heat generated by the exhaust gas is notenough to raise the overall temperature of the filter (e.g., filter witha 2-liter capacity) carrying the NOx catalyst.

Even if control for regeneration of the PMs accumulated on the filter orcontrol for regeneration of the NOx catalyst from sulfur poisoning (alsoreferred to as control for recovery from sulfur poisoning) need becarried out in such a state, it is impossible to raise the temperatureof the NOx catalyst to a temperature range required for such controls.It is therefore impossible to carry out these controls. As a result, thePMs and NOx are not removed, thereby possibly causing insufficientpurification of the exhaust gas.

SUMMARY OF THE INVENTION

The invention is made to solve the above problems, and it is an objectof the invention to provide a technology capable of conducting removalof PMs captured by a filter and sulfur poisoning recovery control of aNOx catalyst even if an internal combustion engine is left in anextremely low-load operation state.

In order to achieve the above object, an exhaust gas purifying devicefor an internal combustion engine according to the invention adopts thefollowing means. More specifically, the exhaust gas purifying deviceincludes a filter, a filter temperature control means, and a sulfurpoisoning recovery control means. The filter carries a NOx absorbentthereon, and is capable of temporarily capturing fine particlescontained in exhaust gas of the internal combustion engine and ofremoving the fine particles by oxidation in a prescribed temperaturerange. The NOx absorbent functions to absorb NOx contained in theexhaust gas when the exhaust gas flowing into the NOx absorbent exhibitsa lean air-fuel ratio and to discharge the absorbed NOx when the exhaustgas flowing into the NOx absorbent exhibits a theoretical air-fuel ratioor a rich air-fuel ratio. The filter temperature control means executesheat-up control of the filter. The sulfur poisoning recovery controlmeans executes control for eliminating sulfur poisoning of the NOxabsorbent. The exhaust gas purifying device is characterized in thatwhen it is determined that the fine-particle removal control byoxidation or/and the sulfur poisoning recovery control is to beexecuted, and the internal combustion engine has been in an extremelylow load state for a predetermined period or more, an engine speed ofthe internal combustion engine is adjusted to a range where atemperature of the filter can be raised by heat-up control, and theheat-up control is then executed by the filter temperature control meansto raise the temperature of the filter to a predetermined value, therebyexecuting the fine-particle removal control by oxidation or/and thesulfur poisoning recovery control for eliminating sulfur poisoning ofthe NOx absorbent.

According to a further aspect of the invention, an exhaust gas purifyingmethod of an exhaust gas purifying device for an internal combustionengine is provided. The exhaust gas purifying device includes a filter,a filter temperature control means, and a sulfur poisoning recoverycontrol means. The filter carries a NOx absorbent thereon, and iscapable of temporarily capturing fine particles contained in exhaust gasof the internal combustion engine and of removing the fine particles byoxidation in a prescribed temperature range. The NOx absorbent functionsto absorb NOx contained in the exhaust gas when the exhaust gas flowinginto the NOx absorbent exhibits a lean air-fuel ratio and to dischargethe absorbed NOx when the exhaust gas flowing into the NOx absorbentexhibits a theoretical air-fuel ratio or a rich air-fuel ratio. Thefilter temperature control means executes heat-up control of the filter.The sulfur poisoning recovery control means executes control foreliminating sulfur poisoning of the NOx absorbent. The exhaust gaspurifying method includes the steps of: when it is determined that thefine-particle removal control by oxidation or/and the sulfur poisoningrecovery control is to be executed, and the internal combustion enginehas been in an extremely low load state for a predetermined period ormore, adjusting an engine speed of the internal combustion engine to arange where a temperature of the filter can be raised by heat-upcontrol; executing the heat-up control by the filter temperature controlmeans to raise the temperature of the filter to a predetermined value;and executing the fine-particle removal control by oxidation or/and thesulfur poisoning recovery control for eliminating sulfur poisoning ofthe NOx absorbent.

The above exhaust gas purifying device for the internal combustionengine and the above exhaust gas purifying method are characterized inthat, if the internal combustion engine has been in an extremely lowload state for a predetermined period or more and removal of fineparticles by oxidation or recovery from sulfur poisoning is required forthe filter, the engine speed of the internal combustion engine isadjusted to execute these processes, and the heat-up control is executedto raise the temperature of the filter to a predetermined value at whichthe aforementioned processes can be conducted. Thereafter, the removalof fine particles by oxidation and the recovery from sulfur poisoningcan be executed.

The air-fuel ratio of the exhaust gas does not refers to a weight ratioof air to fuel contained in mixture introduced into the internalcombustion engine, but a weight ratio of air to fuel contained in gasdischarged to an exhaust passage as a result of combustion of theinternal combustion engine.

For example, “when the internal combustion state is in the extremely lowload state” refers to the case where the internal combustion engine isin an idle state.

“Adjusting the engine speed of the internal combustion engine to therange where the temperature of the filter can be raised” means that, ifthe internal combustion engine in an idle state or in a state close tothe idle state has an engine speed of less than 1,000 rpm, the enginespeed is raised to, e.g., 1,200 rpm or more. This value of the enginespeed varies depending on the state of the internal combustion engineand other operational states.

In this way, the engine speed of the internal combustion engine is firstraised to increase the heat generation amount, and thus shifted to therange where the temperature of the filter can be raised by the heat-upcontrol. Thereafter, the heat-up control can be executed by anycombination of low-temperature combustion, post-injection,VIGOM-injection, and addition of fuel to an exhaust system according toan operational state of the internal combustion engine. For example, theheat-up control for removing the fine particles by oxidation may beexecuted by combination of one or more of the low-temperaturecombustion, the post-injection, the VIGOM-injection, and the addition offuel to the exhaust system, and the heat-up control for recovery fromsulfur poisoning may be executed by combination of the low-temperaturecombustion and the addition of fuel to the exhaust system.

In the heat-up control for removing the fine particles by oxidation, itis preferable to conduct at least the low-temperature combustion when acoolant temperature of the internal combustion engine is equal to orhigher than a predetermined value, and to conduct at least thepost-injection when the coolant temperature of the internal combustionengine is less than the predetermined value.

The low-temperature combustion is preferably conducted in the internalcombustion engine at a weak lean air-fuel ratio in a range of 15 to 19.

The removal of fine particles by oxidation and the recovery from sulfurpoisoning are individually conducted as required. For example, in therecovery from sulfur poisoning, the temperature of the filter must benormally raised to a higher value (at least 600° C.) than that in theremoval of fine particles by oxidation. Therefore, there may be the casewhere the bed temperature of the filter is raised to about 500° C. toconduct only the removal of fine particles by oxidation.

In the removal of fine particles by oxidation, a pressure gauge may bedisposed upstream and downstream of the filter to measure an exhaust gaspressure in an exhaust passage at positions upstream and downstream ofthe filter. When the difference between the measured exhaust gaspressures reaches a predetermined value or more, it is determined thatat least a predetermined amount of fine particles has been accumulatedon the filter. Therefore, it can be determined that the removal of fineparticles by oxidation is required.

Whether the sulfur poisoning recovery control must be conducted or notmay be determined based on the following factors: the total amount offuel supplied to the engine, the amount of fuel added to the filter, theflowing amount of NOx, which is detected by a NOx sensor provideddownstream of the filter, the running distance of a vehicle having aninternal combustion engine mounted thereon, or the like.

In the exhaust gas purifying device for the internal combustion engineaccording to the invention, an active oxygen discharging agent forabsorbing oxygen when an excessive amount of oxygen is present aroundthe active oxygen discharging agent and discharging the absorbed oxygenas active oxygen when an ambient oxygen concentration is decreased maybe carried on the filter. The active oxygen is discharged from theactive oxygen discharging agent when fine particles adhere to thefilter, whereby the fine particles adhering to the-filter can be removedby oxidation with the discharged active oxygen.

The exhaust gas purifying device for the internal combustion engineaccording to the invention thus provides a series of means forconducting removal of fine particles by oxidation or/and sulfurpoisoning recovery of a NOx absorbent which cannot be conducted unlessthe temperature of the filter is raised to a predetermined value in thecase where the internal combustion is left in an extremely low loadoperational state. Therefore, the removal of fine particles accumulatedon the filter and recovery of the sulfur-poisoned NOx catalyst can berealized even in such a situation.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, features, advantages, and technical andindustrial significance of this invention will be better understood byreading the following detailed description of exemplary embodiments ofthe invention, when considered in connection-with the accompanyingdrawings, in which:

FIG. 1 schematically shows the structure of a diesel engine which hasexhaust and intake systems and to which an exhaust gas purifying devicefor an internal combustion engine according to an embodiment of theinvention is applied;

FIG. 2A is a transverse sectional view of a particulate filter that isapplied to the exhaust gas purifying device for the internal combustionengine according to the embodiment;

FIG. 2B is a longitudinal sectional view of the particulate filter inFIG. 2A;

FIG. 3 is a block diagram showing the internal structure of an ECU thatis applied to the exhaust gas purifying device for the internalcombustion engine according to the embodiment;

FIG. 4 shows the relation between the bed temperature of the particulatefilter in FIGS. 2A and 2B and combustion of PMs; and

FIG. 5A and FIG. 5 b are flowcharts for executing heat-up control thatis applied to the exhaust gas purifying device for the internalcombustion engine according to the embodiment of the invention.

DESCRIPTION OF THE EXEMPLARY EMBODIMENTS

In the following description and the accompanying drawings, theinvention will be described in more detail in terms of exemplaryembodiments.

Hereinafter, a specific embodiment of an exhaust gas purifying devicefor an internal combustion engine according to the invention will bedescribed with reference to the drawings. The following descriptiondeals with an exemplary case where the exhaust gas purifying device forthe internal combustion engine according to the invention is applied toa diesel engine for driving a vehicle.

FIG. 1 schematically shows the structure of an internal combustionengine 1 which has intake and exhaust systems and to which the exhaustgas purifying device according to this embodiment is applied.

The internal combustion engine 1 shown in FIG. 1 is a water-cooledfour-cycle diesel engine having four cylinders 2.

The internal combustion engine 1 has fuel injection valves 3 eachinjecting fuel directly into a combustion chamber of a corresponding oneof the cylinders 2. Each of the fuel injection valves 3 is connected toan accumulator (common rail) 4 for accumulating fuel until apredetermined pressure is reached. The common rail 4 is provided with acommon rail pressure sensor 4 a for outputting an electric signalcorresponding to a fuel pressure in the common rail 4.

The common rail 4 communicates with a fuel pump 6 via a fuel supply pipe5. The fuel pump 6 operates using a rotational torque of an output shaft(crank shaft) of the internal combustion engine 1 as a driving source. Apump pulley 6 a attached to an input shaft of the fuel pump 6 isconnected via a belt 7 to a crank pulley 1 a attached to the outputshaft (crank shaft) of the internal combustion engine 1.

In the fuel injection system thus constructed, if a rotational torque ofthe crank shaft is transmitted to the input shaft of the fuel pump 6,the fuel pump 6 discharges fuel at a pressure corresponding to therotational torque transmitted from the crank shaft to the input shaft ofthe fuel pump 6.

The fuel discharged from the fuel pump 6 is supplied to the common rail4 via the fuel supply pipe 5, accumulated in the common rail 4 until thepredetermined pressure is reached, and distributed to the fuel injectionvalves 3 in the cylinders 2. If a driving current is applied to the fuelinjection valves 3, the fuel injection valves 3 are opened. As a result,fuel is injected from each of the fuel injection valves 3 into acorresponding one of the cylinders 2.

An intake branch pipe 8 is connected to the internal combustion engine1. Each branch of the intake branch pipe 8 communicates with thecombustion chamber of a corresponding one of the cylinders 2 via anintake port (not shown).

The intake branch pipe 8 is connected to an intake pipe 9, which isconnected to an air cleaner box 10. An air flow meter 11 and an intaketemperature sensor 12 are attached to the intake pipe 9 downstream ofthe air cleaner box 10. The air flow meter 11 outputs an electric signalcorresponding to the mass of intake air flowing through the intake pipe9. The intake temperature sensor 12 outputs an electric signalcorresponding to the temperature of intake air flowing through theintake pipe 9.

An intake throttle valve 13 for adjusting the flow rate of intake airflowing through the intake pipe 9 is disposed in the intake pipe 9immediately upstream of the intake branch pipe 8. An intake throttleactuator 14 is attached to the intake throttle valve 13. The intakethrottle actuator 14 is composed of a stepper motor and the like anddrives the intake throttle valve 13 in opening and closing directions.

A compressor housing 15 a for a centrifugal supercharger (turbocharger)15 that operates using hydro-dynamic energy of exhaust gas as a drivingsource is disposed in the intake pipe 9 between the air flow meter 11and the intake throttle valve 13. An inter cooler 16 for cooling intakeair that has reached a high temperature as a result of compression inthe compressor housing 15 a is disposed in the intake pipe 9 downstreamof the compressor housing 15 a.

In the intake system thus constructed, intake air that has flown intothe air cleaner box 10 is removed of dust, dirt, or the like by an aircleaner (not shown) in the air cleaner box 10, and then flows into thecompressor housing 15 a via the intake pipe 9.

The intake air that has flown into the compressor housing 15 a iscompressed by the rotation of a compressor wheel, which is fitted in thecompressor housing 15 a. The intake air that has reached a hightemperature as a result of compression in the compressor housing 15 a iscooled in the inter cooler 16 and flows into the intake branch pipe 8 ifnecessary, the intake throttle valve 13 adjusts the flow rate of theintake air. The intake air that has flown into the intake branch pipe 8is distributed to the combustion chamber of each of the cylinders 2 viaa corresponding one of the branches and ignited using fuel injected froma corresponding one of the fuel injection valves 3 as an ignitionsource.

On the other hand, an exhaust branch pipe 18 is connected to theinternal combustion engine 1. Each branch of the exhaust branch pipe 18communicates with the combustion chamber of a corresponding one of thecylinders 2 via an exhaust port (not shown).

The exhaust branch pipe 18 is connected to a turbine housing 15 b of thecentrifugal supercharger 15. The turbine housing 15 b is connected to anexhaust pipe 19, which is connected downstream thereof to a muffler (notshown).

The exhaust pipe 19 extends across a particulate filter (hereinafter,simply referred to as filter) 20 crying an occlusion/reduction-type NOxcatalyst. An exhaust temperature sensor 24 for outputting an electricsignal corresponding to the temperature of exhaust gas flowing throughthe exhaust pipe 19 is attached to the exhaust pipe 19 upstream of thefilter 20.

A differential pressure sensor 37 is provided in order to detect thedifference in pressure in the exhaust pipe 19 between upstream anddownstream sides of the filter 20.

An exhaust throttle valve 21 for adjusting the flow rate of exhaust gasflowing through the exhaust pipe 19 is disposed in the exhaust pipe 19downstream of the filter 20. An exhaust throttle actuator 22 is attachedto the exhaust throttle valve 21. The exhaust throttle actuator 22 iscomposed of a stepper motor and the like and drives the exhaust throttlevalve 21 in opening and closing directions.

In the exhaust system thus constructed, a mixture (burnt gas) burnt ineach of the cylinders 2 of the internal combustion engine 1 isdischarged to the exhaust branch pipe 18 via the exhaust port and thenflows from the exhaust branch pipe 18 into the turbine housing 15 b ofthe centrifugal supercharger 15. The exhaust gas that has flown into theturbine housing 15 b rotates a turbine wheel with the aid of itshydro-dynamic energy. The turbine wheel is rotatably supported in theturbine housing 15 b. In this case, a rotational torque of the turbinewheel is transmitted to the compressor wheel in the compressor housing15 a mentioned above.

Exhaust gas discharged from the turbine housing 15 b flows into thefilter 20 via the exhaust pipe 19. PMs contained in exhaust gas arecollected and noxious gas components contained in exhaust gas areremoved or purified. The exhaust gas whose PMs have been collected bythe filter 20 and whose noxious gas components have been removed orpurified by the filter 20 is discharged into the atmosphere via themuffler. If necessary, the exhaust throttle valve 21 adjusts the flowrate of the exhaust gas.

The exhaust branch pipe 18 and the intake branch pipe 8 communicate witheach other via an exhaust gas recirculation passage (hereinafter,referred to as EGR passage) 25 through which part of the exhaust gasflowing through the exhaust branch pipe 18 is recirculated into theintake branch pipe 8. The EGR passage 25 extends across a flow rateadjusting valve (hereinafter, referred to as EGR valve) 26. The flowrate adjusting valve 26 is composed of an electromagnetic valve and thelike and changes the flow rate of exhaust gas flowing through the EGRpassage 25 (hereinafter, referred to as EGR gas) in accordance with thepower applied.

An EGR cooler 27 for cooling EGR gas flowing through the EGR passage 25is disposed in the EGR passage 25 upstream of the EGR valve 26. The EGRcooler 27 is provided with a coolant passage (not shown), through whichpart of coolant for cooling the internal combustion engine 1 circulates.

In the exhaust gas recirculation mechanism thus constructed, the EGRpassage 25 becomes passable if the EGR valve 26 is opened. Part of theexhaust gas flowing through the exhaust branch pipe 18 flows into theEGR passage 25, flows through the EGR cooler 27, and is introduced intothe intake branch pipe 8.

In this case, heat is exchanged in the EGR cooler 27 between EGR gasflowing through the EGR passage 25 and coolant of the internalcombustion engine 1. As a result, the EGR gas is cooled.

The EGR gas recirculated from the exhaust branch pipe 18 to the intakebranch pipe 8 via the EGR passage 25 mixes with a new air that has flownfrom an upstream portion of the intake branch pipe 8, and is introducedinto the combustion chambers of the cylinders 2.

It is to be noted herein that EGR gas contains inert gas components thatdo not bum by themselves and that have a high thermal capacity, such aswater (H₂O) and carbon dioxide (CO₂). Therefore, the combustiontemperature of a mixture is low if the mixture contains EGR gas. As aresult, the generation amount of nitrogen oxides (NOx) is reduced.

Furthermore, if EGR gas is cooled in the EGR cooler 27, the temperatureof the EGR gas itself drops and the volume thereof is reduced. Thus,when EGR gas is supplied to a certain one of the combustion chambers,the atmospheric temperature in the combustion chamber does notunnecessarily rise, and the amount (volume) of a new air supplied to thecombustion chamber does not unnecessarily decrease either.

In this embodiment, low-temperature combustion is conducted in which theamount of EGR gas is increased during low load operation as compared tonormal operation, and removal of PMs, purification of NOx and heat-upcontrol of the filter 20 are conducted. The low-temperature combustionwill now be described.

As described above, EGR is conventionally used to suppress generation ofNOx. EGR gas has a relatively high specific heat ratio, and a largeamount of heat is required to raise the temperature of the EGR gas.Therefore, the combustion temperature in the cylinders 2 is reduced asthe ratio of the EGR gas in intake air is increased. Since thegeneration amount of NOx is reduced with reduction in combustiontemperature, the discharge amount of NOx can be reduced with increase inthe EGR gas ratio.

However, as the EGR gas ratio is increased, the generation amount ofsoot starts increasing sharply from a certain EGR gas ratio. Therefore,EGR control is normally conducted at an EGR gas ratio that is lower thanthat value.

As the EGR gas ratio is further increased, the amount of soot isincreases shapely as described above. However, there is a peak in thegeneration amount of soot. If the EGR gas ratio is increased beyond thispeak, the generation amount of soot starts being reduced sharply, andfinally soot is hardly generated.

The reason for this is as follows: when the temperature of the fuel andthe gas around the fuel during combustion in the combustion chambers isequal to or lower than a certain value, hydrocarbons (HC) stop growingbefore becoming soot. When the temperature of the fuel and the gasaround the fuel becomes equal to or higher than a certain value,hydrocarbons (HC) rapidly grow to soot.

Accordingly, no soot will be generated if the temperature of the fueland the gas around the fuel during combustion in the combustion chambersis suppressed to at most a value that stops growth of hydrocarbons (HC).In this case, the temperature of the fuel and the gas around the fuel isgreatly affected by the endothermic effect of the gas around the fuelduring combustion of the fuel. Therefore, generation of soot can besuppressed by adjusting the amount of heat absorbed by the gas aroundthe fuel, that is, the EGR gas ratio, according to the amount of heatgenerated by combustion of the fuel.

The EGR gas ratio for the low-temperature combustion is obtained inadvance by experimentation or the like, and a map of the EGR gas ratiois pre-stored in a ROM (Read Only Memory) 352 in an ECU 35 (as shown inFIG. 3). The EGR gas amount is feedback-controlled based on this map.

Hydrocarbons (HC) that have stopped growing before becoming soot can beburnt by using an oxidizing agent or the like carried on the filter 20.

The low-temperature combustion is thus basically conducted by purifyinghydrocarbons (HC) that have stopped growing before becoming soot byusing an oxidizing agent or the like. Accordingly, in the case where theoxidizing agent or the like is inactive, hydrocarbons (HC) aredischarged to the atmosphere without being purified, and it is thereforedifficult to use the low-temperature combustion.

Moreover, it is during relatively low load operation which generates asmall amount of heat by combustion that the temperature of the fuel andthe gas around the fuel during combustion in the cylinders 2 can becontrolled to at most a value that stops growth of hydrocarbons (HC).

Accordingly, since the internal combustion engine 1 remains in a lowrevolution, low load operational state in this embodiment,low-temperature combustion control is conducted when theocclusion/reduction-type NOx catalyst carried on the filter 20 reachesan active region. Whether the occlusion/reduction-type NOx catalyst isin the active region or not can be determined based on an output signalof the exhaust temperature sensor 24 or the like.

In the low-temperature combustion, NOx can thus be reduced and purifiedby supplying hydrocarbons (HC) serving as a reducing agent to theocclusion/reduction-type NOx catalyst while suppressing discharge of PMssuch as soot. The temperature of the filter 20 can be raised by the heatgenerated as a result of the reduction and purification of NOx.

Accordingly, in this embodiment, the bed temperature of the filter 20 israised by conducting the low-temperature combustion as necessary. Inthis case, heat-up control is conducted with the air-fuel ratio varieddepending on a target temperature. In other words, when the targettemperature is high, the heat-up control is conducted at a low air-fuelratio. A desired air-fuel ratio can be obtained by adjusting the amountof EGR gas.

Note that post-injection is conducted in order to obtain a rich air-fuelratio of exhaust gas by injecting fuel in an expansion stroke or exhauststroke after primary injection.

The filter 20 according to this embodiment will now be described. FIGS.2A and 2B show cross-sections of the filter 20. FIG. 2A is a transversesectional view of the filter 20. FIG. 2B is a longitudinal sectionalview of the filter 20.

As shown in FIGS. 2A and 2B, the filter 20 is of so-called wall-flowtype and has a plurality of exhaust gas flow passages 50, 51 extendingparallel to one another. These exhaust gas flow passages are composed ofexhaust gas inflow passages 50 with their downstream ends closed byplugs 52 and exhaust gas outflow passages 51 with their upstream endsclosed by plugs 53. Note that hatched areas in FIG. 2A indicate theplugs 53. Accordingly, the exhaust gas inflow passages 50 and theexhaust gas outflow passages 51 are disposed alternately with theinterposition of thin partitions 54. In other words, the exhaust inflowpassages 50 and the exhaust outflow passages 51 are disposed such thateach of the exhaust gas inflow passages 50 is surrounded by four of theexhaust gas outflow passages 51 and that each of the exhaust gas outflowpassages 51 is surrounded by four of the exhaust gas inflow passages 50.

The filter 20 is made of a porous material such as cordierite. Thus, asindicated by arrows in FIG. 2B, exhaust gas that has flown into theexhaust gas inflow passages 50 flows out into adjacent ones of theexhaust gas outflow passages 51 through surrounding ones of thepartitions 54.

In the embodiment of the invention, carrier layers made of alumina orthe like are formed on a peripheral wall surface of each of the exhaustgas inflow passages 50 and a peripheral wan surface of each of theexhaust gas outflow passages 51, namely, both surfaces of each of thepartitions 54, and on inner wall surfaces of pores formed in thepartitions 54. The occlusion/reduction-type NOx catalyst is carried onthe carrier layers.

Hereinafter, functions of the occlusion/reduction-type NOx catalystcarried on the filter according to this embodiment will be described.

For example, the filter 20 has a carrier made of alumina, and at leastone material selected from an alkine metal such as potassium (K), sodium(Na), lithium (Li), or cesium (Cs), an alkine earth such as barium (Ba)or calcium (Ca), and a rare earth such as lanthanum (La) or yttrium (Y),and a noble metal such as platinum (Pt) are carried on the carrier. Thisembodiment adopts an occlusion/reduction-type NOx catalyst that isconstructed by having barium (Ba) and platinum (Pt) carried on a carriermade of alumina and adding ceria (Ce₂O₃) capable of storing O₂ to thecarrier.

The NOx catalyst thus constructed absorbs nitrogen oxides (NOx)contained in exhaust gas when exhaust gas flowing into the NOx catalystexhibits a high oxygen concentration.

On the other hand, the NOx catalyst discharges the absorbed nitrogenoxides (NOx) if the oxygen concentration of exhaust gas flowing into theNOx catalyst decreases. In this case, if reducing components such ashydrocarbons (HC) and carbon monoxide (CO) exist in exhaust gas, the NOxcatalyst can reduce the nitrogen oxides (NOx) discharged therefrom tonitrogen (N₂).

If the internal combustion engine 1 is in lean-bum operation, exhaustgas discharged from the internal combustion engine 1 exhibits a leanair-fuel ratio and a high oxygen concentration. Thus, the NOx catalystabsorbs nitrogen oxides (NOx) contained in the exhaust gas. However, ifthe internal combustion engine 1 remains in lean-bum operation for along time, the NOx absorption capacity of the NOx catalyst reaches itslimit. As a result, nitrogen oxides (NOx) contained in exhaust gasremain therein without being removed by the NOx catalyst.

Especially in the case of the internal combustion engine 1 constructedas a diesel engine, a mixture of lean air-fuel ratios is burnt in mostoperational ranges, and exhaust gas thus exhibits lean air-fuel ratiosin most operational ranges. Therefore, the NOx absorption capacity ofthe NOx catalyst tends to reach its limit.

Thus, if the internal combustion engine 1 is in lean-bum operation, itis necessary to decrease the concentration of oxygen contained inexhaust gas flowing into the NOx catalyst, increase the concentration ofa reducing agent, and discharge and reduce nitrogen oxides (NOx)absorbed by the NOx catalyst before the NOx absorption capacity of theNOx catalyst reaches its limit.

As methods of thus decreasing the oxygen concentration, addition of fuelto exhaust gas, the aforementioned low-temperature combustion, a shiftof the timing or number of times of fuel injection into the cylinders 2,and the like are conceivable. This embodiment employs a reducing agentsupply mechanism for adding fuel (light oil) serving as a reducing agentto exhaust gas flowing through the exhaust pipe 19 upstream of thefilter 20. The reducing agent supply mechanism adds fuel to exhaust gas,whereby the concentration of oxygen contained in exhaust gas flowinginto the filter 20 is decreased, and the concentration of the reducingagent is increased.

As shown in the figure, the reducing agent supply mechanism is providedwith a nozzle hole that is directed toward the inside of the exhaustbranch pipe 18. The reducing agent supply mechanism has a reducing agentinjection valve 28, a reducing agent supply passage 29, and a shut-offvalve 31. The reducing agent injection valve 28 opens in response to asignal from the ECU 35 and injects fuel. Fuel discharged from the fuelpump 6 is introduced into the reducing agent injection valve 28 throughthe reducing agent supply passage 29. The shut-off valve 31 is disposedin the reducing agent supply passage 29 to shut off the flow of fuel inthe reducing agent supply passage 29.

In such a reducing agent supply mechanism, high-pressure fuel dischargedfrom the fuel pump 6 is supplied to the reducing agent injection valve28 via the reducing agent supply passage 29. The reducing agentinjection valve 28 then opens in response to a signal from the ECU 35,and fuel serving as a reducing agent is injected into the exhaust branchpipe 18.

The reducing agent injected into the exhaust branch pipe 18 from thereducing agent injection valve 28 decreases the oxygen concentration ofexhaust gas that has flown from an upstream portion of the exhaustbranch pipe 18.

The exhaust gas thus formed and exhibiting a low oxygen concentrationflows into the filter 20. Nitrogen oxides (NOx) absorbed by the filter20 are discharged and reduced to nitrogen (N₂).

Then, the reducing agent injection valve 28 closes in response to asignal from the ECU 35, whereby the reducing agent is stopped from beingadded to the exhaust branch pipe 18.

In this embodiment, fuel is added by injection into exhaust gas.However, it is also appropriate that low-temperature combustion forfurther increasing the amount of EGR gas be performed after thegeneration amount of soot has reached its maximum through an increase inthe recirculation amount of EGR gas. Further, it is also appropriatethat fuel be injected from the fuel injection valves 3 in an expansionstroke, an exhaust stroke, or the like of the internal combustion engine1.

The internal combustion engine 1 constructed as described above isprovided with an electronic control unit (ECU) 35 for controlling theinternal combustion engine 1. The ECU 35 controls the operational stateof the internal combustion engine 1 in accordance with an operatingcondition of the internal combustion engine 1 or a driver's request.

Various sensors such as the common rail pressure sensor 4 a, the airflow meter 11, the intake temperature sensor 12, an intake pipe pressuresensor 17, the exhaust temperature sensor 24, a crank position sensor33, a coolant temperature sensor 34, and an accelerator opening sensor36 are connected to the ECU 35 via electric wires. Output signals fromthese sensors are input to the ECU 35.

The fuel injection valves 3, the intake throttle actuator 14, theexhaust throttle actuator 22, the reducing agent injection valve 28, theEGR valve 26, the shut-off valve 31, and the like are connected to theECU 35 via electric wires. The ECU 35 can control these components.

As shown in FIG. 3, the ECU 35 has a CPU (Central Processing Unit) 351,a ROM 352, a RAM (Random Access Memory) 353, a back-up RAM 354, an inputport 356, and an output port 357, which are interconnected by abidirectional bus 350. The ECU 35 also has an A/D converter (A/D) 355connected to the input port 356.

Output signals from sensors designed to output digital signals, such asthe crank position sensor 33, are input to the input port 356. Theseoutput signals are transmitted to the CPU 351 or the RAM 353 via theinput port 356,

Output signals from sensors designed to output analog signals, such asthe common rail pressure sensor 4 a, the air flow meter 11, the intaketemperature sensor 12, the intake pipe pressure sensor 17, the exhausttemperature sensor 24, the coolant temperature sensor 34, and theaccelerator opening sensor 36, are input to the input port 356 via theAID 355. These output signals are transmitted to the CPU 351 or the RAM353 via the input port 356.

The output port 357 is connected to the fuel injection valves 3, theintake throttle actuator 14, the exhaust throttle actuator 22, the EGRvalve 26, the reducing agent injection valve 28, the shut-off valve 31,and the like via electric wires. Control signals output from the CPU 351are transmitted to the fuel injection valves 3, the intake throttleactuator 14, the exhaust throttle actuator 22, the EGR valve 26, thereducing agent injection valve 28, and the shut-off valve 31 via theoutput port 357.

The ROM 352 stores application programs such as a fuel injection controlroutine for controlling the fuel injection valves 3, an intake throttlecontrol routine for controlling the intake throttle valve 13, an exhaustthrottle control routine for controlling the exhaust throttle valve 21,an EGR control routine for controlling the EGR valve 26, an NOxpurification control routine for discharging absorbed NOx by adding areducing agent to the filter 20, a poisoning elimination control routinefor eliminating SOx poisoning of the filter 20, and a PM combustioncontrol routine for burning and removing the PMs collected by the filter20.

In addition to the aforementioned application programs, the ROM 352stores various control maps. For example, the control maps include afuel injection amount control map showing the relation betweenoperational states of the internal combustion engine 1 and base fuelinjection amounts (base fuel injection periods), a fuel injection timingcontrol map showing the relation between operational states of theinternal combustion engine 1 and base fuel injection timings, an intakethrottle valve opening control map showing the relation betweenoperational states of the internal combustion engine 1 and targetopenings of the intake throttle valve 13, an exhaust throttle valveopening control map showing the relation between operational states ofthe internal combustion engine 1 and target openings of the exhaustthrottle valve 21, an EGR valve opening control map showing the relationbetween operational states of the internal combustion engine 1 andtarget openings of the EGR valve 26, a reducing agent addition amountcontrol map showing the relation between operational states of theinternal combustion engine 1 and target addition amounts of the reducingagent (or target air-fuel ratios of exhaust gas), a reducing agentinjection valve control map showing the relation between target additionamounts of the reducing agent and opening periods of the reducing agentinjection valve 28, and the like.

The RAM 353 stores output signals from the sensors, calculation resultsobtained from the CPU 351, and the like. For example, the calculationresults include an engine speed that is calculated based on a timeinterval at which the crank position sensor 33 outputs a pulse signal.These data are updated every time the crank position sensor 33 outputs apulse signal.

The back-up RAM 354 is a non-volatile memory capable of holding dataeven after the internal combustion engine 1 is stopped.

The CPU 351 operates in accordance with the application programs storedin the ROM 352 and performs fuel injection valve control, intakethrottle control, exhaust throttle control, EGR control, NOxpurification control, poisoning elimination control, PM combustioncontrol, and the like.

For example, during NOx purification control, the CPU 351 performsso-called rich spike control in which the concentration of oxygencontained in exhaust gas flowing into the filter 20 is decreased in aspike manner on a relatively short cycle (in a short period).

In rich spike control, the CPU 351 determines on a predetermined cyclewhether or not a condition for performing rich spike control has beenfulfilled. For example, this condition for performing rich spike controlis that the filter 20 has been activated, that the output signal valueof the exhaust temperature sensor 24 (exhaust gas temperature) is equalto or smaller than a predetermined upper limit value, that poisoningelimination control is not being performed, or the like.

If it is determined that the condition for performing rich spike controlas described above has been fulfilled, the CPU 351 controls the reducingagent injection valve 28 so as to inject fuel serving as a reducingagent from the reducing agent injection valve 28 in a spike manner.Thus, the CPU 351 temporarily makes the air-fuel ratio of exhaust gasflowing into the filter 20 equal to a predetermined target rich air-fuelratio.

More specifically, the CPU 351 reads an engine speed stored in the RAM353, an output signal of the accelerator opening sensor 36 (acceleratoropening), an output signal value of the air flow meter 11 (intake airamount), an output signal of the air-fuel ratio sensor, a fuel injectionamount, and the like.

Using the engine speed, the accelerator opening, the intake air amount,and the fuel injection amount as parameters, the CPU 351 accesses thereducing agent addition amount control map stored in the ROM 352 andcalculates an addition amount (target addition amount) of the reducingagent required to make the air-fuel ratio of exhaust gas equal to apreset target air-fuel ratio.

Using the target addition amount as a parameter, the CPU 351 thenaccesses the reducing agent injection valve control map stored in theROM 352 and calculates an opening period (target opening period) of thereducing agent injection valve 28 required to inject the target additionamount of the reducing agent from the reducing agent injection valve 28.

If the target opening period of the reducing agent injection valve 28 iscalculated, the CPU 351 opens the reducing agent injection valve 28.

If the target opening period has elapsed after the opening of thereducing agent injection valve 28, the CPU 351 closes the reducing agentinjection valve 28.

If the reducing agent injection valve 28 is thus opened for the targetopening period, the target addition amount of fuel is injected from thereducing agent injection valve 28 into the exhaust branch pipe 18. Thereducing agent injected from the reducing agent injection valve 28 mixeswith exhaust gas that has flown from an upstream portion of the exhaustbranch pipe 18, forms a mixture having the target air-fuel ratio, andflows into the filter 20.

As a result, the oxygen concentration of exhaust gas flowing into thefilter 20 changes on a relatively short cycle. Thus, the filter 20repeats the absorption of nitrogen oxides (NOx) and thedischarge/reduction of nitrogen oxides (NOx) alternately on a shortcycle.

In poisoning elimination control, the CPU 351 performs a poisoningelimination process so as to eliminate poisoning of the filter 20 byoxides.

It is to be noted herein that the internal combustion engine 1 may use afuel containing sulfur (S). If such a fuel bums in the internalcombustion engine 1, sulfur oxides (SOx) such as sulfur dioxide (SO₂)and sulfur troxide (SO₃) are produced.

Sulfur oxides (SOx) flow into the filter 20 together with exhaust gasand are absorbed by the filter 20 according to the same mechanism as inthe case of nitrogen oxides (NOx).

More specifically, if exhaust gas flowing into the filter 20 exhibits ahigh oxygen concentration, sulfur oxides (SOx) contained in the exhaustgas, such as sulfur dioxide (SO₂) and sulfur trioxide (SO₃), areoxidized on the surface of platinum (Pt) and are absorbed by the filter20 in the form of sulfate ions (SO₄ ²⁻). The sulfate ions (SO₄ ²⁻) thusabsorbed by the filter 20 bond to barium oxide (BaO) and form bariumsulfate (BaSO₄).

It is to be noted herein that barium sulfate (BaSO₄) is stabler and lesslikely to be decomposed than barium nitrate (Ba(NO₃)₂). Even if theoxygen concentration of exhaust gas flowing into the filter 20decreases, barium sulfate (BaSO₄) remains in the filter 20 without beingdecomposed.

If the amount of barium sulfate (BaSO₄) in the filter 20 increases, theamount of barium oxide (BaO) that can contribute to the absorption ofnitrogen oxides (NOx) decreases accordingly. This leads to so-calledsulfur poisoning, which causes deterioration in the NOx absorptioncapability of the filter 20.

According to one exemplary method for eliminating sulfur poisoning ofthe filter 20, the atmospheric temperature of the filter 20 is raised toa high temperature range of about 600 to 650° C., and the oxygenconcentration of exhaust gas flowing into the filter 20 is decreased. Asa result, barium sulfate (BaSO₄) absorbed by the filter 20 is thermallydecomposed into SO₃ ⁻ and SO₄ ⁻. Then, SO₃ ⁻ and SO₄ ⁻ are caused toreact with hydrocarbons (HC) and carbon monoxide (CO) contained inexhaust gas and reduced to gaseous SO₂ ⁻.

Thus, the poisoning recovery process according to this embodiment isdesigned such that the CPU 351 first performs catalyst heat-up controlfor raising the bed temperature of the filter 20 and then decreases theoxygen concentration of exhaust gas flowing into the filter 20.

In catalyst heat-up control, the CPU 351 may be designed, for example,to inject fuel from each of the fuel injection valves 3 secondarilyduring an expansion stroke of a corresponding one of the cylinders 2,add the fuel to exhaust gas from the reducing agent injection valve 28to oxidize unburnt components of the fuel in the filter 20, and raisethe bed temperature of the filter 20 by means of heat generated throughthe oxidation.

However, if the filter 20 is heated up excessively, thermal degradationof the filter 20 may be induced. It is therefore preferable to performfeedback control of the secondary injection amount of fuel and theaddition amount of fuel based on an output signal value of the exhausttemperature sensor 24.

If the bed temperature of the filter 20 rises to a high temperaturerange of about 600 to 650° C. through the aforementioned catalystheat-up process, the CPU 351 causes fuel to be injected from thereducing agent injection valve 28 so as to decrease the oxygenconcentration of exhaust gas flowing into the filter 20.

If an excessive amount of fuel is injected from the reducing agentinjection valve 28, the fuel may bum in the filter 20 abruptly andoverheat the filter 20. Otherwise, the filter 20 may be cooledunnecessarily by the excessive amount of fuel injected from the reducingagent injection valve 28. It is therefore preferable that the CPU 351perform feedback control of the fuel injection amount from the reducingagent injection valve 28 based on an output signal of an air-fuel ratiosensor (not shown).

If the poisoning recovery process is thus performed, the oxygenconcentration of exhaust gas flowing into the filter 20 decreases underthe condition that the bed temperature of the filter 20 is high. Then,barium sulfate (BaSO₄) absorbed by the filter 20 is thermally decomposedinto SO₃ ⁻ and SO₄ ⁻. The SO₃ ⁻ and SO₄ ⁻ react with hydrocarbons (HC)and carbon monoxide (CO) contained in exhaust gas and are reduced,whereby the filter 20 is recovered from the sulfur poisoning.

Hereinafter, a flow of heat-up control and sulfur poisoning recoverycontrol according to this embodiment will be described.

FIGS. 5A and 5B are flowcharts for executing heat-up control accordingto this embodiment. This control is started if removal of fine particlesby oxidation (hereinafter, referred to as PM recovery or PMregeneration) or recovery from sulfur poisoning is to be performed, thatis, if a flag indicating that these controls are to be executed is ON.

The sulfur poisoning recovery control is started based on the total fuelconsumption, an output signal from a NOx sensor (not shown), a vehiclerunning distance, and the like. Since sulfur components contained infuel poison the occlusion/reduction-type NOx catalyst carried on thefilter 20, the total fuel consumption may be stored in the RAM 353 andthe sulfur poisoning recovery control may be started when the additionamount of fuel reaches a predetermined value. As the sulfur poisoningproceeds, the amount of NOx absorbed by the occlusion/reduction-type NOxcatalyst decreases and the amount of NOx flowing downstream of thefilter 20 increases. Therefore, a NOx sensor (not shown) may be disposeddownstream of the filter 20. In this case, an output signal of the NOxsensor may be monitored, and the sulfur poisoning recovery control maybe started when the amount of NOx flowing downstream of the filter 20reaches a predetermined value or more. Moreover, when the vehiclerunning distance reaches a predetermined value or more, it is determinedthat recovery from sulfur poisoning is required, and a sulfur poisoningrecovery control flag is set.

In the case of PM recovery, if the difference in pressure in the exhaustpipe 19 between the upstream and downstream sides of the filter 20,which is detected by the differential pressure sensor 37, reaches apredetermined value or more, it can be estimated that at least aprescribed amount of PMs has been accumulated on the filter 20. Thus, aPM recovery control flag is set if at least a prescribed amount of PMsis accumulated.

If the sulfur poisoning recovery flag or the PM recovery flag is ON, theroutine proceeds to step S101.

In step S101, it is determined whether the internal combustion engine 1is in a low load state or not. If the internal combustion engine 1 isnot in the low load state, it is determined that PM recovery and thelike need not be performed based on the heat-up control. Therefore, theroutine is terminated.

On the other hand, if the internal combustion engine 1 is in the lowload state, the routine proceeds to step S102. In step S102, it isdetermined whether or not the bed temperature of the filter 20 is lessthan 150° C. The temperature of the filter 20 is estimated by using theexhaust temperature sensor 24 disposed in the exhaust pipe 19immediately upstream of the filter 20. If the temperature of the filter20 is less than 150° C., the catalyst is not activated and effectiveexhaust purification cannot be carried out.

If the bed temperature of the filter 20 is less than 150° C., theroutine proceeds to step S103. If the bed temperature of the filter 20is equal to or higher than 150° C., the control is terminated. In thiscase, removal of fine particles by oxidation and recovery from sulfurpoisoning are performed according to normal heat-up control and thelike.

In step S103, it is determined whether or not the internal combustionengine 1 has been left in the low load state and the bed temperature ofthe filter 20 has been held at less than 150° C. for a predeterminedperiod (first predetermined period) or more. The first predeterminedperiod is determined in view of various factors. For example, the firstpredetermined period may be fifteen minutes.

If the internal combustion engine 1 has been left in the low load stateand the bed temperature of the filter 20 has been held at less than 150°C. for the first predetermined period or more, the routine proceeds tostep S104. Otherwise, the control is terminated.

In step S104, it is determined whether the internal combustion engine 1is in an idle state or not. For example, if the engine speed of theinternal combustion engine 1 is about 750 rpm, it is determined that theinternal combustion engine 1 is in the idle state, and the routineproceeds to step S106. Otherwise, the routine proceeds to step S105, andearly warm-up combustion is conducted.

In step S106, the engine speed of the internal combustion engine 1 israised to 1,200 rpm and the early warm-up combustion is conducted. Theroutine then proceeds to step S107.

The early warm-up combustion is conducted in order to raise thetemperature of the filter 20. Hereinafter, exemplary methods of theearly warm-up combustion will be described.

A first exemplary method is to retard the fuel injection timing to thetop dead center of a compression stroke or later during combustion ofthe internal combustion engine 1. In normal combustion, primary fuel isinjected near the top dead center of a compression stroke. If theinjection timing is retarded, an after burning period is increased,whereby the exhaust gas temperature rises. The temperature of the filter20 rises with the rise of the exhaust gas temperature.

A second exemplary method is to inject secondary fuel near the top deadcenter of an intake stroke in addition to the primary fuel. Hereinafter,such additional injection of the secondary fuel is referred to asVIGOM-injection. The VIGOM-injection increases the fuel injectionamount. Therefore, the exhaust gas temperature rises, whereby thetemperature of the filter 20 can be raised.

The VIGOM-injection near the top dead center of an intake strokeproduces intermediate products such as aldehyde, ketone, peroxide andcarbon monoxide by the compression heat during a compression stroke.These intermediate products accelerate reaction of the primary fuel thatis injected subsequently. In this case, misfire will not occur even ifthe injection timing of the primary fuel is retarded, whereby excellentcombustion is realized. The exhaust gas temperature can thus be raisedby retarding the injection timing of the primary fuel. Therefore, thetemperature of the filter 20 can be raised.

A third exemplary method is to conduct post-injection during anexpansion stroke or an exhaust stroke in addition to injection of theprimary fuel. In this case, most of the fuel injected by thepost-injection is discharged to the exhaust pipe in the form of unburnthydrocarbons (HC). The unburnt hydrocarbons (HC) are oxidized on thefilter 20. The temperature of the filter 20 is raised by heat generatedthrough the oxidation.

In step S107, it is determined whether the bed temperature of the filter20 is at least 180° C. or not. If the bed temperature is 180° C. orhigher, the routine proceeds to step S108.

On the other hand, if the bed temperature is less than 180° C., theroutine returns to step S106, and the early warm-up combustion iscontinued while maintaining the engine speed of 1,200 rpm.

In step S108, it is determined whether the coolant temperature is atleast 60° C. or not. If the coolant temperature is 60° C. or higher, theroutine proceeds to step S109, and weak lean low-temperature combustionis conducted as a means for raising the temperature of the filter 20.The weak lean low-temperature combustion is conducted at the air-fuelratio close to 18. However, the air-fuel ratio is not limited to 18 andmay be in the range of 17 to 19. The low-temperature combustion isconducted at an air-fuel ratio that is lower than the air-fuel ratio inthe normal combustion of the internal combustion engine 1. Sincehydrocarbon (HC) components contained in the fuel bum on the filter 20,the bed temperature of the filter 20 rises.

Note that the air-fuel ratio can be adjusted to a desired value byvarying the amount of EGR gas.

On the other hand, if the coolant temperature is less than 60° C., theroutine proceeds to step S111, and post-injection is conducted as ameans for raising the temperature of the filter 20.

These different means for raising the temperature of the filter 20 arethus used depending on the coolant temperature in order to stabilizecombustion of the internal combustion engine 1. In other words, if thecoolant temperature is 60° C. or higher, the low-temperature combustionis stable to raise the temperature of the filter 20. If the coolanttemperature is less than 60° C., however, the low-temperature combustionbecomes unstable. The reason why the low-temperature combustion isselected if the coolant temperature is 60° C. or higher is as follows:in the low load state, only a small amount of fuel is injected by thepost-injection. In the case where such a small amount of fuel isinjected in a plurality of times, it is difficult to control theinjection amount. It is therefore preferable to conduct heat-up controlby the low-temperature combustion.

It is thus preferable to conduct heat-up control by the low-temperaturecombustion because combustion stability is more likely to be obtained.If the coolant temperature is less than 60° C., however, post-injectionis preferred to the low-temperature combustion in order to maintain anexcellent combustion state.

Note that, it is desirable not to conduct the heat-up control of thefilter 20 by the addition of fuel when the filter 20 is in a lowbed-temperature range. This is in order to prevent the added fuel havinga low temperature from adhering to the wall surface of the exhaust pipe.It is therefore preferable to employ the low-temperature combustion orthe post-injection as a means for heat-up control that is conductedherein, because the low-temperature combustion and the post-injection donot cause such a problem.

After the low-temperature combustion is conducted in step S109, it isdetermined in step S110 whether the bed temperature of the filter 20 isat least 300° C. or not.

Even if the post-injection is conducted in step S111, it is determinedin step S112 whether the bed temperature of the filter 20 is at least300° C. or not.

If the bed temperature is less than 300° C. in step S110, the routinereturns to step S109 and the weak lean low-temperature combustion iscontinued.

If the bed temperature is less than 300° C. in step S112, the routinereturns to step S111 and the post-injection is continued.

If the bed temperature is 300° C. or higher in step S110, the routineproceeds to step S114, and the weak lean low-temperature combustion iscontinued at a further reduced air-fuel ratio than the air-fuel ratio inthe low-temperature combustion. The weak lean low-temperature combustionof step S114 is conducted at an air-fuel ratio close to 16. However, theair-fuel ratio is not limited to 16 and may be in the range of 15 to 17.

If the bed temperature is 300° C. or higher in step S112, the routineproceeds to step S113, and it is determined whether the coolanttemperature is at least 60° C. or not. If the coolant temperature is 60°C. or higher, the post-injection is discontinued. The routine thenproceeds to step S114, and the weak lean low-temperature combustion isconducted.

If the bed temperature is less than 60° C. in step S113, the routineproceeds to step S115, and the fuel is added to the exhaust system forheat-up control.

After the low-temperature combustion of step S114 is conducted for aprescribed period, the routine proceeds to step S116, and it isdetermined whether the bed temperature of the filter 20 has reached atleast 500° C. or not.

If the fuel is added in step S115, the routine proceeds to step S117after a predetermined period, and it is determined whether the bedtemperature of the filter 20 has reached at least 500° C. or not.

If the bed temperature of the filter 20 has not reached 500° C. in stepS116, the routine returns to step S114, and the weak leanlow-temperature combustion is further continued.

If the bed temperature of the filter 20 has not reached 500° C. in stepS117, the routine returns to step S115, and the fuel is further added.

If the bed temperature of the filter 20 is 500° C. or higher in stepS116, it is determined that PM recovery has been completed when thedifference in pressure detected by the differential pressure sensor 37has reduced to a predetermined value or less. If recovery from sulfurpoisoning is not required, that is, if the sulfur poisoning recoveryflag is OFF, the control is terminated.

Similarly, if the bed temperature of the filter 20 is 500° C. or higherin step S117, PM recovery is continued. It is determined that the PMrecovery has been completed when the difference in pressure detected bythe differential pressure sensor 37 has reduced to a predetermined valueor less. If recovery from sulfur poisoning is not required, that is, ifthe sulfur poisoning recovery flag is OFF, the control is terminated.

On the other hand, if the sulfur poisoning recovery flag is ON, theroutine proceeds to step S118, and the sulfur poisoning recovery controlis executed. In this case, weak lean low-temperature combustion andaddition of fuel are conducted in order to raise the temperature of thefilter 20 to 600° C.

The routine then proceeds to step S119, and it is determined whether thebed temperature of the filter 20 is at least 600° C. or not. If the bedtemperature is 600° C. or higher, the routine proceeds to step S122, andit is determined whether the sulfur poisoning recovery control has beenconducted for a second predetermined period or more, e.g., three minutesor more. On the other hand, if the bed temperature is less than 600° C.,the routine proceeds to step S120, and it is determined whether theair-fuel ratio is lower than the stoichiometric air-fuel ratio(theoretical air-fuel ratio) or not. If the air-fuel ratio is leanerthan the stoichiometric air-fuel ratio, the routine returns to stepS118, and heat-up control for recovery from sulfur poisoning iscontinued. On the other hand, if the air-fuel ratio is equal to orricher than the stoichiometric air-fuel ratio, the routine proceeds tostep S121, and addition of fuel is discontinued for a prescribed period.In this state, it is estimated that oxygen does not exist in exhaustgas. Therefore, the fuel will not bum even if the fuel is further added.Accordingly, the routine returns to step S118 after a predeterminedperiod that allows oxygen to exist in the exhaust gas. In step S118, thelow-temperature combustion and addition of fuel are conducted in orderto continue the heat-up control for recovery from sulfur poisoning.

If the sulfur poisoning recovery control has been conducted for thesecond predetermined period or more, that is, three minutes or more intotal in step S122, it is determined that the catalyst has beenrecovered from sulfur poisoning, and the control is terminated.

If the sulfur poisoning recovery control has not been conducted forthree minutes or more in total in step S122, this control is furthercontinued. The control is terminated when it has been conducted forthree minutes or more in total.

As has been described above, the exhaust gas purifying device for theinternal combustion engine according to this embodiment operates asfollows: removal of fine particles by oxidation or/and sulfur poisoningrecovery control may be required in the state where the internalcombustion engine remains in an idle state for a prescribed period, thatis, the internal combustion engine remains in an extremely low loadstate for a predetermined period or more. In this case, the aboveexhaust gas purifying device first adjusts the engine speed of theinternal combustion engine (1) to a range where the temperature of thefilter (20) can be raised by the heat-up control. The exhaust gaspurifying device then conducts the heat-up control by a filtertemperature control means to raise the temperature of the filter (20) toa predetermined value. In order to raise the temperature of the filter,one or more appropriate methods out of low-temperature combustion,post-injection, addition of fuel to the exhaust system, and the like areconducted in combination depending on the state such as an operationalstate of the internal combustion engine, coolant temperature, and thelike. When the filter (20) reaches the predetermined temperature, theexhaust gas purifying device conducts removal of fine particles byoxidation or/and sulfur poisoning recovery control for eliminatingsulfur poisoning of a NOx absorbent

The exhaust gas purifying device for the internal combustion engineaccording to the present invention is capable of carrying aNOx-purifying catalyst thereon and raising the temperature of the filtercapable of capturing PMs to a predetermined temperature range even ifthe internal combustion engine remains in an extremely low loadoperational state. Therefore, removal of the PMs captured by the filterand sulfur poisoning recovery control of the NOx catalyst can bereliably conducted even in such a situation.

Removal of fine particles by oxidation or/and sulfur poisoning recoverycontrol may be required when an internal combustion engine has been inan extremely low load state for a predetermined period or more. In thiscase, the engine speed of the internal combustion engine 1 is adjustedto a range where the temperature of a filter 20 can be raised by heat-upcontrol. The heat-up control is then executed by a filter temperaturecontrol means to raise the temperature of the filter 20 to apredetermined value. When the filter 20 reaches the predeterminedtemperature by means of low-temperature combustion, post-injection,VIGOM-injection, addition of fuel to an exhaust system and the like,removal of fine particles by oxidation or/and sulfur poisoning recoverycontrol for eliminating sulfur poisoning of a NOx absorbent areconducted. Removal of PMs captured by the filter and sulfur poisoningrecovery control of the NOx absorbent can thus be conducted even if theinternal combustion engine is left in an extremely low load operationalstate.

While the invention has been described with reference to exemplaryembodiments thereof, it is to be understood that the invention is notlimited to the exemplary embodiments or constructions. To the contrary,the invention is intended to cover various modifications and equivalentarrangements. In addition, while the various elements of the exemplaryembodiments are shown in various combinations and configurations, whichare exemplary, other combinations and configurations, including more,less or only a single element, are also within the spirit and scope ofthe invention.

1. An exhaust gas purifying device for an internal combustion engine,comprising: a filter carrying a NOx absorbent thereon and capable oftemporarily capturing fine particles contained in an exhaust gas of theinternal combustion engine and of removing the fine particles byoxidation in a prescribed temperature range, the NOx absorbentfunctioning to absorb NOx contained in the exhaust gas when the exhaustgas flowing into the NOx absorbent exhibits a lean air-fuel ratio and todischarge the absorbed NOx when the exhaust gas flowing into the NOxabsorbent exhibits one of a theoretical air-fuel ratio and a richair-fuel ratio; and a controller that: executes heat-up control of thefilter; executes sulfur poisoning recovery for eliminating sulfurpoisoning of the NOx absorbent, and adjusts an engine speed of theinternal combustion engine to a range where a temperature of the filtercan be raised by heat-up control (1) when it is determined thatfine-particle removal by oxidation is to be executed or the sulfurpoisoning recovery control is to be executed, (2) when it is determinedthat the temperature of the filter is below a predetermined temperatureand (3) when it is determined that the internal combustion engine hasbeen in an extremely low load state for a predetermined period or more,and then executes the heat-up control to raise the temperature of thefilter to a predetermined value, thereby executing fine-particle removalby oxidation or executing the sulfur poisoning recovery control foreliminating sulfur poisoning of the NOx absorbent, wherein: the heat-upcontrol is executed by at least one of low-temperature combustion,post-injection, VIGOM-injection, and addition of fuel to an exhaustsystem according to an operational state of the internal combustionengine, the heat-up control for removing the fine particles by oxidationis executed by at least one of the low-temperature combustion, thepost-injection, the VIGOM-injection, and the addition of fuel to theexhaust system, and the heat-up control for recovery from sulfurpoisoning is executed by a combination of the low-temperature combustionand the addition of fuel to the exhaust system, and in the heat-upcontrol for removing the fine particles by oxidation, at least thelow-temperature combustion is conducted when a coolant temperature ofthe internal combustion engine is equal to or higher than apredetermined value, and at least the post-injection is conducted whenthe coolant temperature of the internal combustion engine is less thanthe predetermined value.
 2. An exhaust gas purifying method of anexhaust gas purifying device for an internal combustion engine,including a filter carrying a NOx absorbent thereon and capable oftemporarily capturing fine particles contained in an exhaust gas of theinternal combustion engine and of removing the fine particles byoxidation in a prescribed temperature range, the NOx absorbentfunctioning to absorb NOx contained in the exhaust gas when the exhaustgas flowing into the NOx absorbent exhibits a lean air-fuel ratio and todischarge the absorbed NOx when the exhaust gas flowing into the NOxabsorbent exhibits one of a theoretical air-fuel ratio and a richair-fuel ratio, the method comprising the steps of: adjusting an enginespeed of the internal combustion engine to a range where a temperatureof the filter can be raised by heat-up control, (1) when it isdetermined that fine-particle removal by oxidation is to be executed orsulfur poisoning recovery control is to be executed, (2) when it isdetermined that the temperature of the filter is below a predeterminedtemperature and (3) when it is determined that the internal combustionengine has been in an extremely low load state for a predeterminedperiod or more; executing the heat-up control to raise the temperatureof the filter to a predetermined value; and executing fine-particleremoval by oxidation or executing the sulfur poisoning recovery controlfor eliminating sulfur poisoning of the NOx absorbent, wherein: theheat-up control is executed by at least one of low-temperaturecombustion, post-injection, VIGOM-injection, and addition of fuel to anexhaust system according to an operational state of the internalcombustion engine, the heat-up control for removing the fine particlesby oxidation is executed by at least one of the low-temperaturecombustion, the post-injection, the VIGOM-injection, and the addition offuel to the exhaust system, and the heat-up control for recovery fromsulfur poisoning is executed by a combination of the low-temperaturecombustion and the addition of fuel to the exhaust system, and in theheat-up control for removing the fine particles by oxidation, at leastthe low-temperature combustion is conducted when a coolant temperatureof the internal combustion engine is equal to or higher than apredetermined value, and at least the post-injection is conducted whenthe coolant temperature of the internal combustion engine is less thanthe predetermined value.